1. Field of the Invention
This invention relates to high-purity lanthanum isopropoxide and a process for producing the same.
2. Description of the Related Art
The lanthanum isopropoxide La(OiC3H7)3 (referred to hereinafter as La(OiPr)3) is useful as a starting material for asymmetric synthetic catalyst or as a starting material for fine particles or film containing lanthanum oxide. Shibasaki et al. found that a La—Na-BINOL catalyst obtained by reacting La(OiPr)3 with optically active binaphthol [referred to hereinafter as BINOL] and sodium tertiary butoxide is useful for asymmetric Michael reaction (JP-A 8-291178). Similarly, a La—K-BINOL catalyst is useful for asymmetric hydrophoshonylation (JP-A 8-325281) while a La—Li-BINOL catalyst is useful for asymmetric Mannich reaction (JP-A 2000-72727). Further, a La-BINOL catalyst is useful for asymmetric epoxidation reaction (JP-A 10-120668).
It is empirically found that the catalytic performance thereof is influenced by the producing process and physical properties of La(OiPr)3, but the cause therefor is not elucidated. Accordingly, there is the need for further excellent La(OiPr)3 as a starting material for the catalyst giving higher reproducibility.
Known processes for producing La(OiPr)3 include:    (1) LaCl3+3Na(OiPr)=La(OiPr)3+3NaCl    (2) LaCl3.3iPrOH+3nBuLi=La(OiPr)3.3LiCl+nBuH    (3) La(OOCR)3+3Na(OiPr)=La(OiPr)3+3Na(OOCR)    (4) La+3iPrOH=La(OiPr)3+3/2H2 
In the process (1), La(OiPr)3 was obtained by reacting the starting materials in equivalent amounts in boiling isopropanol (S. N. Misra, T. N. Misra, R. N. Kapoor and R. C. Mehrotra, Chemistry & Industry, 120 (1963)). The literature does not refer to the analytical contents of impurities, alkali metals and Cl, in the synthesized La(OiPr)3. There are few literatures describing the specific analytical contents of impurities, alkali metals and Cl, and only JP-A 6-1737 discloses the contents of Na and Cl.
In the process (2), A. Lebrun et al. in Tetrahedron Letters, vol. 32, 2355 (1991) describe that LiCl coordinates to La(OiPr)3 and it is hard to be purified.
JP-B 62-6694 concerning the process (3) does not describe the content of La or the amount of impurity Na.
In the direct method (4), there are no impurities such as alkali metals and Cl. L. M. Brown and K. S. Mazdiyasni in Inorg. Chem. Vol. 9, 2783 (1970) describe that La(OiPr)3 can be synthesized by reacting La with isopropanol directly in the presence of a HgCl2 catalyst, but there is no specific description such as sublimation data, La content, and yield, although there is a description of data on isopropoxide with 13 other lanthanoids. The present inventors confirmed that the La content is often higher than the theoretical content, possibly because compounds having La—O—La bonds (e.g. LaxOy(OiPr)z) have been formed. The La content in Comparative Example 4 was as high as 112% based on the theoretical content.
When the oxide film on the surface of the La metal is strong, the induction time is prolonged, and the reaction time is thus prolonged, and as a result, the product may be easily deteriorated. Further, the catalyst used is a Hg salt, so the prior art process is not preferable as an industrial process.
JP-A 6-1737 describes that when Na(OiPr) is charged in excess, the product La(OiPr)3 cannot be separated even by repeated re-crystallization from toluene, possibly because the excess starting material acts together with the product to form a complex. It is therefore described therein that because LaCl3 is insoluble in toluene, use of the other starting material LaCl3 in slight excess is very effective in eliminating the Na content and in reducing the reaction time. In this case, Na is absent, but Cl remains. In such case, Cl is estimated to be contained in compounds such as La6(OiPr)17Cl. This is the present inventors' estimation because R. A. Andersen et al. in Inorg. Chem. Vol. 17, 1962 (1978) synthesized and identified Nd6(OiPr)17Cl.
Further, it is noted that Y(OiPr)3 is often identified with Y5O(OiPr)13 or contains Y5O(OiPr)13depending on the synthesis process, synthesis conditions and purification method, and O. Poncelet et al. in Inorg. Chem. Vol. 28, 263 (1989) synthesized and identified Y5O(OiPr)13. This can also apply to Ln(OiPr)3 wherein Ln is a lanthanoid, and it is noted that the product partially contains Ln5O(OiPr)13.
There is no report on the La(OiPr)3 system, but when the La content is higher than the theoretical content of 43.9 wt-%, the product is assumed to contain compounds such as La5O(OiPr)13. This compound contains La—O—La bonds besides La—O—iPr. That is, pure La(OiPr)3 must be an aggregate of La(OiPr)3, which consists of a “n”-mer represented by [La(OiPr)3]n. The pure compound must not contain the La—O—La oxide. In high-purity La(OiPr)3, the amount of alkali metals, Cl, and La—O—La oxide must be very low. However, there are few literatures on measurement of “n” or structural analysis of the aggregate, and if any, there are doubts about reliability. The present inventors estimated that the major reason therefor is that the substance measured was not real [La(OiPr)3]n.
N. I. Kozlova et al. in Koord. Khim. Vol. 8, 639 (1982) had concluded that because in mass spectrometry of La(OiPr)3 crystals obtained by direct synthesis method in the production process (4), the strongest spectrum at the maximum m/Z was attributable to La5(OiPr)11O+, the product was a pentamer, that is, [La(OiPr)3]5.
However, it seems that this substance was not the real La(OiPr)3 because its melting point described therein was 120 to 128° C. The present inventors estimated that this substance is La5O(OiPr)13.niPrOH (n=2). This is because the low melting point suggests coordination of isopropanol thereto and the La content suits well with the theoretical value of 43.5% for the coordinated compound.
On one hand, S. N. Misra, T. N. Misra, R. N. Kapoor and R. C. Mehrotra in Chemistry & Industry (London) 120 (1963) had reported that La(OiPr)3 sublimed at 250 to 300° C./0.1 Torr, and occurred as a monomer in benzene. According to R. C. Mehrotra and J. M. Batwara in Inorg. Chem. Vol. 9, 2505 (1970), the lowest sublimation temperature of La(OiPr)3 was 250 to 280° C./0.01 Torr. However, if this substance is a monomer, the substance should sublimed easily at 200° C. or less because of its low molecular weight, so the reference thereof to as a monomer is not reliable.
D. C. Bradley, R. C. Mehrotra and D. P. Gaur in “Metal Alkoxides” (Academic Press, 1978) p. 104 have estimated the structure of lanthanoid isopropoxide, assuming that it occurs inherently as a tetramer. However, lanthanoid elements are significantly different from each other in ionic radius and coordination number, so the properties of La(OiPr)3 can be different from those of other counterparts. They have not measured or discussed La(OiPr)3. As described above, the degree of association and structure of the real La (OiPr)3 have not been definitely determined. In the lanthanoid group, La(OiPr)3 is the most indefinite compound.
There are few literatures on synthesis of La(OiPr)3 where the La content was analyzed. In JP-A 6-1737, the La content in Example 1 is 43.7 wt-% which is certainly near to the theoretical value, but Na content is 2.3 wt-%, so when OiPr in Na(OiPr) is subtracted therefrom (that is, OiPr=54.1−(59/23)×2.3=48.2 wt-%), OiPr of 48.2 wt-% is bound to La. That is, OiPr mole/La mole=(48.2/59)/(43.7/138.9)=2.60, which is considerably lower than the theoretical value of 3. This value is significantly deviated from the theoretical value, even assuming that the analysis accuracy of La is ±2%. This is probably because compounds having La—O—La bonds are contained as impurities in a considerable amount.
On the other hand, in Example 4 wherein Na is absent and Cl is 0.4 wt-%, Cl mole/La mole=(0.4/35.45)/(44.1/138.9)=0.04, indicating that there are 4% La—Cl bonds.
In the Examples and Comparative Examples in JP-A 6-1737, La(OiPr)3 of highest purity contains La, 44.1%; Na, not described; and Cl, 0.4%. The content of Na (not described) assumed from analysis accuracy is that Na<about 0.1%. There were no other literatures describing the analytical contents of the 3 elements La, Na and Cl. That is, La(OiPr)3 of highest purity in the prior art contains La, 44.1%; Na<0.1%; and Cl, 0.4%.
There are no literatures referring to the behavior of synthesis reaction of La(OiPr)3 by using K(OiPr) in place of Na(OiPr) or to the La content in the product and the amount of K and Cl impurities.